Mothers who lose too much weight quickly after giving birth could be putting their next baby at risk, doctors have warned.

On the other hand, gaining too much weight after a pregnancy can be just as dangerous.

Any rapid weight changes between pregnancies can increase the risk of babies suffering high blood pressure, along with the prospect of premature or stillbirth.

The warning comes from two studies highlighted yesterday by the British Medical Journal.

The researchers, from Coombe Women's Hospital, Dublin, urged women to maintain a healthy weight before, during and after pregnancy to give their children the best start in life.

Many women struggle to lose the extra weight they gain during pregnancy, despite following advice to exercise, eat healthily and breast feed where possible.

Others slim down so quickly they end up thinner than they were before becoming pregnant.

The pressure to lose weight after having a baby is heightened by celebrities such as Victoria Beckham and Catherine Zeta-Jones who appeared to return to their pre-pregnancy shapes within weeks.

Dr Jennifer Walsh and Dr Deirdre Murphy said few women were aware the effect fluctuating weight had on unborn children.

There is growing evidence that sudden and dramatic changes in a mother's weight puts a huge strain on the body and can harm babies.

One study, published last year in the American Journal of Obstetrics and Gynaecology, examined the risks of losing too much weight between babies.

It found that women whose BMI fell by five or more units between pregnancies had a higher risk of premature birth than women whose weight remained stable.

The effect was heightened among those who had already experienced one premature birth.

Another study from Sweden published last year, examined the risks of gaining too much weight.

It looked at 207,500 women between 1992 and 2001 and linked changes in body mass index with a baby and mother's health.

BMI takes into account weight and height to give an indicator of how obese or underweight a person is. A BMI over 25 is classified as overweight, under 20 is underweight and over 30 is obese.

The study found a rise in a woman's BMI by just one or two points between the start of her first pregnancy and the start of her second doubled the risks of diabetes and pre-eclampsia - a disease of pregnancy that can lead to dangerously high blood pressure.

Gaining weight equivalent to more than three BMI points ' significantly increased the rate of still birth, independent of obesity related diseases', the experts said.

The risks increased for both overweight women and those who were slim, the researchers found.

Dr Walsh and Dr Murphy said it was important for women not to lose or gain weight too quickly or dramatically.

Their report said: 'Although apparently conflicting, these studies show how important it is to attain and maintain a normal healthy weight before, during, and after pregnancy.

'Pregnancy is one of the most nutritionally demanding periods of a woman's life, with an adequate supply of nutrients essential to support foetal wellbeing and growth.

'With at least half of all pregnancies unplanned, women need to be aware of the implications of their weight for pregnancy, birth, and the health of their babies.'

Tam Fry from the National Obesity Forum, said: 'I think these doctors are absolutely right.'

He said it was 'absolutely fundamental' that girls were taught a healthy weight was necessary not only for themselves but for the wellbeing of their future children.

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